HTML5 is a new web standard that is gaining popularity rapidly and adds many new features to your web experience. Most notably for YouTube users, HTML5 includes support for video and audio playback. This means that users with an HTML5 compatible browser, and support for the proper audio and video codecs can watch a video without needing to download a browser plugin.
Our support for HTML5 is an early experiment, and there are some limitations. HTML5 on YouTube doesn’t support videos with ads, captions, or annotations and it requires a browser that supports both the video tag and h.264 encoded video (currently that means Chrome, Safari, and ChromeFrame on Internet Explorer). We will be expanding the capabilities of the player in the future, so get ready for new and improved versions in the months to come.